1. Field of the Invention
The invention relates to apparatus and methods for carrying out electrodeionization to purify water, and more particularly to an electrodeionization device and method with improved scaling resistance.
2. Description of Related Art
Electrodeionization (EDI) is a membrane separation deionization technique that combines the techniques of electodialysis and ion exchange. EDI purification apparatus has many advantages, such as, producing water continuously, regenerating ion exchange resins without using alkalis and acids, automatically operating, etc. It has become a standard alternative to mixed bed as the final water treatment apparatus used in pure water preparation systems. A plate and frame type EDI apparatus includes an anode, a cathode, anion-permeable membranes and cation-permeable membranes. The membranes are arranged alternately in such a manner as to alternately form concentrating compartments and desalting compartments (dilution compartments) in a stack between the anode and the cathode. The desalting compartments are filled with an ion exchanger such as an ion exchange resin beads. The liquid being treated in the diluting compartments is depleted of ions while the liquid in the concentrating compartments becomes enriched with the transferred ions through their respective membrane and carries them in concentrated form.
The cations and anions ions in the feed water to the EDI apparatus can perform ion exchange with the H+ and OH− in the cation and anion exchange resins respectively, and therefore are ionically attach to the resin particles. The ions migrate under the influence of electric field through the ion-flow passage formed by resin particles. This is because that in the application systems of EDI, the electric conductivity of the resin is several magnitudes higher than that of the water solution. The ions migrate into the concentrate chamber through the ion exchange membranes, and hence complete the process of water deionization. Under a certain potential drop, the water is decomposed into H+ and OH− due to the assisted water dissociation at the interface of the two different types of resins and membranes and the resin is therefore regenerated.
The diluting compartments are filled with porous ion exchanging solid materials producing voids between the particles through which the water to be deionized flows. The ion exchanging materials are commonly mixtures of cation exchanging resins and anion exchanging resins or woven and non-woven fibers. An assembly of one or more pairs of diluting and concentrating compartments, referred to as a “cell pair”, is bounded on either side by an anode and a cathode which typically apply an electric field perpendicular to the general direction of liquid flow. However, in other configurations, the current and liquid flow in the same or opposite directions. The applied electric field causes anions to move from the diluting compartment across the anion exchange membrane into the concentrating compartment nearer the anode and cations to move from the diluting compartment across the cation exchange membrane into the concentrating compartment nearer the cathode. The anions and cations become trapped in the concentrating compartments because the movement of anions toward the anode is blocked by a cation exchange membrane, and the movement of cations toward the cathode is blocked by an anion exchange membrane. A flow of water is set up to remove the ions from the concentrating compartments. The net result of the process is the removal of ions from the water stream flowing through the diluting compartments and their concentration in the water flowing through the concentrating compartments.
Typically, the EDI feed water is initially pretreated in a reverse osmosis step to reduce the ionic load and colloidal contaminants therein, prior to being directed towards electrodeionization. This practice extends the useful life of the resin beads used in electrodeionization. However, even when using a reverse osmosis pretreating step, the concentration of calcium and/or magnesium cations and sulfate and/or carbonate anions can cause so-called “scaling” in the concentration compartments due to precipitation. The consequence of this scaling is restricted concentrate flow, an increase in stack electrical resistance, a drop in current density and eventually a sharp decrease in the purity of the product water. This negatively affects performance characteristics by increasing operating cost, decreasing product water quality, or making the EDI stack inoperable.
It is desired to have an electrodeionization device and method with improved scaling resistance.